A known image forming apparatus for transferring a toner image formed on the surface of an image carrier onto a recording medium nipped in a transfer nip is disclosed in Japanese Patent Application Laid-open No. 2006-267486 (hereinafter, Patent Document 1). The image forming apparatus disclosed in Patent Document 1 forms a toner image on the surface of a drum-shaped photosensitive element functioning as an image carrier through a known electrophotographic process. An endless intermediate transfer belt that is an image carrier as an intermediate transfer body abuts against the photosensitive element, and a primary transfer nip is thus formed. The toner image formed on the photosensitive element is then primarily transferred onto the intermediate transfer belt in the primary transfer nip. A secondary transfer roller as a transfer member abuts against the intermediate transfer belt, and a secondary transfer nip is thus formed. A secondary transfer facing roller is arranged inside of the loop of the intermediate transfer belt, and the intermediate transfer belt is nipped between the secondary transfer facing roller and the secondary transfer roller. The secondary transfer facing roller arranged inside of the loop is grounded. A secondary transfer bias (voltage) is applied from a power supply to the secondary transfer roller arranged outside of the loop. In this manner, a secondary transfer field for electrostatically transferring the toner image from the secondary transfer facing roller to the secondary transfer roller is formed between the secondary transfer facing roller and the secondary transfer roller, that is, in the secondary transfer nip. The toner image on the intermediate transfer belt is then secondarily transferred onto a recording sheet fed into the secondary transfer nip at operational timing synchronized with the toner image on the intermediate transfer belt, by the effects of the secondary transfer field and a nipping pressure.
In such a structure, when a recording sheet with a highly textured surface such as washi (Japanese paper) is used, density patterns following the texture of the surface could be more easily formed in an image. These density patterns are caused because a sufficient amount of toner is not transferred onto recessed parts of the paper surface, and the image density in the recessed parts becomes thin compared with that in projected parts. In response to this issue, the image forming apparatus disclosed in Patent Document 1 is structured to apply a superimposed bias in which a direct current voltage is superimposed over an alternating current voltage, besides a direct current voltage, as the secondary transfer bias. In Patent Document 1, by applying such a secondary transfer bias, formations of density patterns are suppressed compared with when a secondary transfer bias consisting only of a direct current voltage is applied.
However, experiments conducted by inventors of the present invention have revealed that, in the conventional technology described above, when the secondary transfer bias is applied in the manner disclosed in Patent Document 1, a plurality of white spots tend to be formed more easily in an image at locations corresponding to the recessed parts of the paper surface.
An object of the present invention is to provide an image forming apparatus and an image forming method for suppressing formations of white spots and achieving high quality images, while obtaining sufficient image densities in both of the recessed parts and the projected parts of a recording medium surface.